On the surface of the grain oriented electrical steel sheet, coating is generally applied for the purpose of imparting insulation properties, workability, rust resistance and the like. Such coating comprises a base film mainly composed of forsterite formed during final annealing and a phosphate-based top coating formed thereon.
These coatings are formed at a high temperature, and have a low thermal expansion coefficient. Therefore, when the steel sheet temperature is lowered to room temperature, tension resulting from the difference between the thermal expansion coefficient of the steel sheet and those of the coatings is imparted to the steel sheet. This tension provides an effect of reducing iron loss, and therefore it is desirable to impart as much tension as possible to the steel sheet.
To satisfy such demands, various types of coatings have been conventionally proposed. For example, JPS5652117B (PTL 1) describes a coating mainly composed of magnesium phosphate, colloidal silica, and chromic anhydride. Further, JPS5328375B (PTL 2) describes a coating mainly composed of aluminum phosphate, colloidal silica, and chromic anhydride.
Meanwhile, due to the growing interest in environmental preservation in recent years, there has been an increasing demand for products containing no harmful substances such as chromium, lead and the like. There has been a demand for development of coating containing no chromium i.e. chromium-free coating in the field of grain oriented electrical steel sheets as well. However, chromium-free coating has low moisture absorption resistance and poor tension imparting performance.
As methods for resolving the above problems, coating formation methods using treatment solutions containing colloidal silica, aluminum phosphate, boric acid, and sulfate were proposed in JPS54143737B (PTL 3) and JPS579631B (PTL 4). With these methods, it is possible to improve characteristics of the coating, i.e. the moisture absorption resistance and the iron loss reduction effect obtained by imparting tension to some degree. However, the characteristics were insufficient compared to conventional coating containing chromium.
Under the situation, various methods were proposed for the purpose of further improving coating characteristics. For example, an attempt was made for a method of increasing the amount of colloidal silica contained in the treatment solution for forming the coating. With said method, the tension imparting performance of the obtained coating was improved. However, the moisture absorption resistance decreased.
An attempt was also made for a method of increasing the additive amount of sulfate. However, with this method, although the moisture absorption resistance of the coating was improved, the tension imparting performance decreased, and a sufficient iron loss reduction effect could not be obtained. As described above, neither of the methods could improve both moisture absorption resistance and tension imparting performance to the necessary level.
As chromium-free coating formation methods other than the above, a method of adding a boric acid compound instead of a chromium compound has been proposed in JP2000169973A (PTL 5), a method of adding an oxide colloid has been proposed in JP2000169972A (PTL 6), and a method of adding a metal organic acid salt has been proposed in JP2000178760A (PTL 7).
However, even by using any of these techniques, it was not possible to enhance both the moisture absorption resistance and the iron loss reduction effect obtained by imparting tension, to the same level as conventional coating containing chromium, and these techniques could not be perfect solutions.
Further, JP200723329A (PTL 8) and JP200957591A (PTL 9) describe techniques similar in some respects to that of the disclosure. PTL 8 describes a technique of containing metallic elements such as Fe, Al, Ga, Ti, Zr and the like in the treating solution for forming the coating for the purpose of preventing hydration. PTL 9 describes a technique of improving moisture absorption resistance of the coating by adding Ti chelate to the treatment solution for forming the coating.